Method for making a mold for casting metallic melts

ABSTRACT

The invention relates to a method for producing a mould for casting metallic melts, in particular for casting titanium, titanium alloys or intermetallic titanium aluminides. Said method consists of the following steps: a contact layer is produced by applying a first slicker containing a first metal oxide powder as an essentially solid component to a moulded core, a first sanding layer is produced on the contact layer formed from the first slicker by sanding with a second metal oxide powder and the layer sequence formed from the contact layer and the first sanding layer is radiated with infrared light for a predetermined period of time. According to the invention, for speeding up the drying process, a first dry mass of the first slicker contains a hydraulic binder.

The invention relates to a method for making a mold for casting metallicmelts, in particular for casting titanium, titanium alloys orintermetallic titanium aluminides as defined by the preamble of claim 1.

Such a method is known from EP 1 645 348 B1. According to the knownmethod, it is provided that IR radiation is used to support the dryingof a slicker layer applied onto a mold core. The drying of the slickerlayer can be accelerated with this.

With these kinds of molds, a plurality of slicker and sanding layers areapplied one after the other. It is thereby necessary respectively to dryeach newly applied layer sequence of slicker. and sanding layers beforethe next layer sequence is applied. The drying takes longer the morelayer sequences there are. The making of a mold with several layersequences requires in total a great amount of time of more than 3 hours.

The object of the invention is to specify a method with which the timerequired to make molds for the casting of metallic melts can be furtherreduced.

This object is solved by the features of claim 1. Useful embodimentsresult from the features of claims 2 to 28.

According to the invention it is provided that a first dry mass formaking the first slicker contains a hydraulic binder. Hydraulic bindersset when absorbing water. They thus withdraw water from the contactlayer and harden this at the same time. The firmness of the contactlayer is increased by providing the hydraulic binder. Due to this, themold can be built up with a smaller number of layer sequences. Inparticular, this can significantly speed up the making of a mold made upof a plurality of layer sequences.

In the sense of the present invention, a “hydraulic binder” isunderstood to mean a mixture of substances which hydrates with water.The hydration products formed by this cause the solid componentscontained in the slicker to harden.

According to an advantageous embodiment, the first dry mass contains 1to 30 wt. %, preferably 8 to 17 wt. %, particularly preferably 9 to 13wt. %, of the hydraulic binder. The hydraulic binder is usefully acalcium aluminate cement. Such a hydraulic binder is chemically inertfor the most part. When a calcium aluminate cement is used, molds can bemade with which highly reactive metallic melts, in particularintermetallic titanium aluminide melts can be cast. It has thereby beenproven to be advantageous that 1 to 15 wt. % of the hydraulic binder iscontained in the first dry mass.

According to a further advantageous embodiment, a grain band of thefirst metal oxide powder has a range of 0 to 50 μm and advantageously amedium grain size (d₅₀) in the range of 8 to 20 μm. The second metaloxide powder used to make the first sanding layer can have a mediumgrain size in the range from 130 to 200 μm. A layer sequence made fromthe contact layer and the first sanding layer is then dried inter aliawith infrared light for a specified amount of time. The hydraulic binderprovided by the invention contributes to an improved hardening of themold.

According to a further embodiment, it is provided that the first and/orsecond :metal oxide powder is formed from at least one metal oxide whichis selected from the following group: Y₂O₃, CeO, MgO, Al₂O₃. The firstdry mass can thereby usefully contain less than 95 wt. %, preferablyless than 90 wt. %, of the first metal oxide powder. The first dry massusefully contains 40 to 70 wt. % of the first metal oxide powder.

If Y₂O₃ is used as the first metal oxide powder, an excellent resistanceto highly reactive metallic melts, in particular intermetallic titaniumaluminide melts, can be achieved. Such a mold is particularly suitablefor making titanium aluminide components. In this case, the first drymass usefully contains at the most 15 wt. % of the hydraulic binder. Thefirst dry mass is usefully free of MgO—except for unavoidableimpurities. This can be used to achieve a particularly good resistanceto the previously mentioned highly reactive metallic melts.

According to a further embodiment feature, a coating layer is madesurrounding the layer sequence from contact and first sanding layer orseveral layer sequences. The coating layer can contain MgO as theessential component. Moreover, a second dry mass for making the coatinglayer can contain a hydraulic binder, preferably calcium aluminatecement. The calcium aluminate cement advantageously contains 60 to 80wt. % of Al₂O₃, preferably approximately 70 wt. % of Al₂O₃. A second drymass usefully contains at least 40 wt. % of MgO, preferably 60 to 80 wt.%, as well as at least 20 wt. % of the hydraulic binder. Moreover, thesecond dry mass can contain at least 1 wt. % of one or more of thefollowing oxides: Fe₂O₃, SiO₂, CaO, Al₂O₃. The coating layer isessentially used to mechanically stabilize the layer sequence. It canhave a significantly greater layer thickness than the layer sequence. Amold having the suggested coating layer is particularly suitable forusing the centrifugal casting method.

It has been proven to be particularly useful to apply an intermediatelayer sequence formed from an intermediate and a second sanding layeronto the layer sequence formed from contact and first sanding layerbefore making the coating layer. This can improve the thermo shockresistance of the mold.

The contact layer is usefully applied using the injection method. Theintermediate layer can also be applied using the injection method oralso using the dipping method. By applying the contact and/orintermediate layer using the injection method, the respective layers canbe applied with a particularly low layer thickness and improved surfacequality at the same time. This can further accelerate the drying of therespective layers.

The second slicker can contain a first MgO powder as the essential solidcomponent. Moreover, the second slicker can contain a hydraulic binder,preferably a calcium aluminate cement. The calcium aluminate cement canin turn contain 60 to 80 wt. % of Al₂O₃, preferably approximately 70 wt.% of Al₂O₃.

A third dry mass for making the second slicker can contain at least 50wt. % MgO, preferably 60 to 70 wt. %, and at least 20 wt. % of thehydraulic binder. The second sanding layer is usefully made by applyingthe second MgO powder onto the intermediate layer.

There can be several such intermediate layer sequences, each of which isformed from an intermediate layer and a second sanding layer, appliedone after the other onto the layer sequence of contact and first sandinglayers. Several layer sequences of contact and first sanding layers canalso be provided. The second sanding layer is usefully made by applyinga second MgO powder onto the intermediate layer. The first MgO powdercan thereby have a smaller medium grain size than the second MgO powder.

The suggested intermediate layer sequence contributes to an improvedthermo shock resistance of the mold. When the intermediate layers alsocontain a hydraulic binder, they can also be made quickly andefficiently, particularly using the dipping method.

With regard to the respective application, it has proven useful that thefirst and/or third dry mass and/or the sanding layer/layerscontains/contain at least 1 wt. % of one of the following oxides: CeO₂,La₂(D₃, Gd₂O₃, Nd₂O₃, TiO₂. The addition of other oxides of the rareearths is also possible.

According to a particularly advantageous embodiment, a moisture contentof the layer and/or the intermediate layer sequence is reduced to aspecified value after it is made. The specified value can be in therange from 10% to 60% of the residual moisture. It is preferably lessthan 20%, but more than 5% residual moisture. In other words, thecontact and/or intermediate layer is advantageously not completely driedbefore fore applying the first or second sanding layer. Thissignificantly accelerates the making of the mold. Surprisingly, it hasbeen shown that the suggested retention of a residual moisture alsoresults in molds having an excellent firmness. When the suggestedresidual moisture is retained when making the mold, the number ofintermediate layers and second sanding layers can be significantlyreduced or such layers do not even need to be provided. When the mold ismade with retention of the suggested residual moisture, surprisinglysuch a good firmness of the contact and/or intermediate layer can beachieved that also here the addition of a hydraulic binder is notneeded. To this extent, a method for making a mold using the setting ofthe previously mentioned residual moisture in the contact and/orintermediate layer is viewed as an independent invention. With such amethod, infrared radiation can also be used for drying or setting theresidual moisture.

In case a hydraulic binder is used, it must be ensured that a sufficientresidual moisture is retained in the layer sequence or the intermediatelayer sequence so that the hydraulic binder is able to completely set.The water consumption caused by the setting of the hydraulic bindercontributes to an acceleration of the drying and firming process of therespective layer or intermediate layer sequence as well as of thecoating layer.

A drying time or a time for setting the residual moisture is usefullyless than 25 minutes, preferably 5 to 15 minutes per layer orintermediate layer sequence.

In particular with regard to the mechanical and chemical stability ofthe mold as well as its efficient manufacturing, it has proven to beuseful that the fraction of the hydraulic binder in the first dry massis less than that in the second or third dry mass. The fraction ofhydraulic binder in the second and/or third dry mass is advantageouslyabout at least 2 wt. %, preferably at least 5 wt. %, greater than in thefirst dry mass.

Furthermore, it has proven to be useful that the first and/or secondslicker has/have a viscosity of 1000 mPas at the most, preferablybetween 450 and 750 mPas. Slickers with such a viscosity can beparticularly well processed using the injection method.

According to further embodiment features of the method, after thecoating layer has been made, the mold core is removed by melting orburning out the material forming the mold core. The material is usefullywax or similar. Melting the material forming the mold core and/or anadditional drying of the layer and/or intermediate layer sequence canalso be accomplished with microwaves.

A green body formed after the removal of the mold core is usefullysintered at a sintering temperature of more than 800° and less than1200° C. The suggested use according to the invention of a hydraulicbinder to make the mold contributes to a significant reduction of thesintering temperature in comparison to methods as per prior art.

Examples will now be used to describe the invention in more detail.

The sole figure schematically shows a partial cross sectional viewthrough a mold according to the invention. A contact layer 1 contains 85wt. % Y₂O₃ and 15 wt. % of hydration products of the calcium aluminatecement, for example. Reference sign 2 designates a first sanding layerwhich is composed essentially of a further Y₂O₃ powder having a mediumgrain size of approximately 150 μm. The contact layer 1 and the firstsanding layer 2 form one layer sequence A.

An intermediate layer 3 is applied onto the first sanding layer 2, whichintermediate layer 3 contains MgO as the essential component which inturn is set by the reactive product of a calcium aluminate cement.Reference sign 4 designates a second sanding layer which is made from anMgO powder. Reference sign B designates an intermediate layer sequencemade of alternating layers of intermediate layer 3 and second sandinglayer 4. The intermediate layer sequence B is lined on the back with acoating layer 5 which contains MgO as the essential solid componentwhich in turn is set by a hydraulic binder, preferably calcium aluminatecement.

Similar to the intermediate layer sequence B, the layer sequence A canalso be formed from a sequence of several contact layers 1 as well asfirst sanding layers 2 in alternating sequence.

EXAMPLE 1

To make the contact layer 1, a first slicker is made first whose firstdry mass contains 80 to 90 wt. % of a first Y₂O₃ powder. The mediumgrain size d₅₀ of the Y₂O₃ powder is usefully 10 to 15 μm. The modalvalue is advantageously 15 to 25 μm. The grain band is usefully locatedin the range between 0 and 50 μm. Moreover, the first dry mass usefullycontains 10 to 20 wt. % of a calcium aluminate cement. Adding a suitableamount of water creates a first slicker having a viscosity in the rangefrom 400 to 700 mPas. The first slicker is injected using the injectionmethod onto a mold core made, for example, of wax. Then the contactlayer 1 made from the first slicker is sanded with a first sanding layer2. The first sanding layer 2 consists essentially of a second Y₂O₃powder which has a medium grain size in the range from 170 to 200 μm.The thus made layer sequence A is then dried, for example under IRaction for a time period of approximately 10 to 20 minutes, preferablyup to a residual moisture of 10 to 30%.

The previously mentioned method for making the contact layer 1 as wellas the first sanding layer 2 can be repeated multiple times, for examplethree to five times. The layer sequence A can also be concluded by acontact layer 1 instead of the first sanding layer 2. It can also bethat a first sanding layer concluding the layer sequence A isessentially made from an MgO powder. With regard to its grain sizedistribution as well as its medium grain size, this MgO powder can bemade like the second Y₂O₃ powder.

A second slicker is made to make one or more intermediate layers 3 ofthe intermediate layer sequence B. A second dry mass for making thesecond slicker contains, for example, 65 to 80 wt. %, preferably 65 to70 wt. %, MgO as well as 20 to 35 wt. %, preferably 30 to 35 wt. % of acalcium aluminate cement. By adding a suitable amount of water, a secondslicker is made whose viscosity is usefully set in such a manner that itenables a coating using the conventional dipping method. To make theintermediate layer 3, the layer sequence A applied onto the mold core iscoated with the second slicker using the dipping method. Afterwards, thesecond sanding layer 4 is applied which is essentially formed from anMgO powder having a grain size in the range from 0.1 to 2.0 mm. Aftermaking the second sanding layer 4, the thus formed intermediate layersequence B is usefully dried for a duration of 15 to 20 minutes under IRaction. Also a residual moisture in the range from 10 to 30% can be setin the intermediate layer sequence B. Then further intermediate 3 andsecond sanding layers 4 can be applied in the same way. Finally, theintermediate layer sequence B can be dried for a time period of 10 to 25minutes under IR action.

To make the coating layer 5, a still flowable mass is made from 60 to 80wt. %, preferably 70 to 80 wt. % of MgO, 20 to 40 wt. %, preferably 20to 30 wt. % of calcium aluminate cement as well as water and auxiliarysubstances. The coated mold core is then placed in a mold and theflowable mass is poured over it. After drying the mass preferably underIR action, the mold core is removed by increasing the temperature. Thenthe thus formed green body is sintered at a temperature in the rangefrom 1000 to 1200° C., preferably in the range between 1100 and 1200° C.

EXAMPLE 2

In this case, a first dry mass for making the first slicker contains 60to 70 wt. % of Y₂O₃ and 10 to 20 wt. % of CeO₂. The grain band of themixture is between 0 and 50 μm. Moreover, the first dry mass contains 10to 20 wt. % of the calcium aluminate cement. The first dry mass is mixedwith water so that a first slicker having a viscosity in the range from300 to 600 mPas is formed.

Otherwise, the method is performed as discussed in example 1.

The drying of the layer sequence A and/or the intermediate layersequence B can be supported by using IR radiation. The coated mold corecan, preferably in addition, be led through a drying chamber in whichthe air temperature is in the range from 30 to 60° C.

In addition to the previously mentioned components, the first and/orsecond slicker can also contain conventional auxiliary substances, inparticular the organic auxiliary substances in the usual amounts.

1. Method for making a mold for casting metallic melts, in particularfor casting titanium, titanium alloys or intermetallic titaniumaluminides, with the following steps: making a contact layer by applyinga first slicker onto a mold core, which first slicker contains a firstmetal oxide powder as an essential solid component, making a firstsanding layer on the contact layer formed from the first slicker bysanding with a second metal oxide powder, irradiating the layer sequenceformed from the contact layer and the first sanding layer with infraredlight for a specified period of time, characterized in that a first drymass of the first slicker contains a hydraulic binder.
 2. Method asdefined in claim 1, wherein the first dry mass contains 1 to 30 wt. %,preferably 8 to 17 wt. % of the hydraulic binder.
 3. Method as definedin claim 1, wherein the hydraulic binder is a calcium aluminate cement.4. Method as defined in claim 1, wherein a grain band of the first metaloxide powder has a range from 0 to 50 μm and advantageously a mediumgrain size (d₅₀) in the range from 8 to 20 μm.
 5. Method as defined inclaim 1, wherein the second metal oxide powder has a medium grain sizein the range from 130 to 200 μm.
 6. Method as defined in claim 1,wherein the first and/or second metal oxide powder is formed from atleast one metal oxide which is selected from the following group: Y₂O₃,CeO, MgO, Al₂O₃.
 7. Method as defined in claim 1, wherein the first drymass contains less than 90 wt. % of the first metal oxide powder. 8.Method as defined in claim 1, wherein a coating layer surrounding alayer sequence of contact and first sanding layer is made.
 9. Method asdefined in claim 1, wherein the coating layer contains MgO as theessential component.
 10. Method as defined in claim 1, wherein a seconddry mass for making the coating layer contains a hydraulic binder,preferably a calcium aluminate cement.
 11. Method as defined in claim 1,wherein a second dry mass contains at least 40 wt. % MgO as well as atleast 20 wt. % of the hydraulic binder.
 12. Method as defined in claim1, wherein the second dry mass contains at least 1 wt. % of one or moreof the following oxides: Fe₂O₃, SiO₂, CaO, Al₂O₃.
 13. Method as definedin claim 1, wherein before making the coating layer, an intermediatelayer sequence formed from an intermediate layer and a second sandinglayer is applied onto the layer sequence formed from a first contact andfirst sanding layer.
 14. Method as defined in claim 1, wherein thecontact and/or the intermediate layer are applied using the injectionmethod.
 15. Method as defined in claim 1, wherein a second slickercontains a first MgO powder as the essential solid component.
 16. Methodas defined in claim 1, wherein the second slicker contains a hydraulicbinder, preferable a calcium aluminate cement.
 17. Method as defined inclaim 1, wherein a third dry mass for making the second slicker containsat least 50 wt. % of MgO and at least 20 wt. % of the hydraulic binder.18. Method as defined in claim 1, wherein the second sanding layer ismade by applying a second MgO powder onto the intermediate layer. 19.Method as defined in claim 1, wherein the first and/or third dry massand/or the sanding layer/layers contains/contain at least 1 wt. % of oneof the following oxides: CeO₂, La₂O₃, Gd₂O₃, Nd₂O₃, TiO₂.
 20. Method asdefined in claim 1, wherein a moisture content of the layer and/or theintermediate layer sequence is reduced to a specified value after theyare made.
 21. Method as defined in claim 1, wherein the specified valueis in the range from 10 to 60% residual moisture, preferably less than20% residual moisture.
 22. Method as defined in claim 1, wherein adrying time per layer or intermediate layer sequence is less than 25minutes, preferably 5 to 15 minutes.
 23. Method as defined in claim 1,wherein the fraction of the hydraulic binder in the first dry mass isless than in the second or third dry mass.
 24. Method as defined inclaim 1, wherein the fraction of the hydraulic binder in the secondand/or third dry mass is greater than in the first dry mass by at least2 wt. %, preferable by at least 5 wt. %.
 25. Method as defined in claim1, wherein the first and/or second slicker has/have a viscosity of notmore than 1000 mPas, preferably between 450 and 750 mPas.
 26. Method asdefined in claim 1, wherein, after making the coating layer, the moldcore is removed by melting or burning out the material forming the moldcore.
 27. Method as defined in claim 1, wherein melting the materialforming the mold core and/or an additional drying of the layer and/orintermediate layer sequence is performed using microwaves.
 28. Method asdefined in claim 1, wherein a green body formed after the removal of themold core is sintered at a sintering temperature of more than 800° C.and less than 1200° C.